The present invention relates generally to digital communications systems and more particularly relates to a highly stable, fast tracking equalizer suitable for use in combating time varying channels within a digital communications system.
In the coming years digital television broadcasting will take on more and more of a dominant role in television broadcasting. Public broadcasting of digital television signals has already begun the United States. Some time in the early 2000s, it is forecasted that the broadcasting of most analog television signals will cease and will be replaced by TV signals that are digital in nature, whether over terrestrial links, i.e., over the air, cable or satellite.
A problem associated with the transmission of terrestrial digital TV signals is signal reflections. Signal reflections can be caused by many factors including stationary objects such as buildings and moving objects such as airplanes.
Weak reflections that are generated relatively close to the receiver do not pose as nearly as much of a problem as strong reflections that are generated relatively far away from the receiver. The latter type of reflections being very problematic to the reception of digital signals such as digital TV signals.
Channel reflections are another common form of linear distortion ISI which constitute a common impairment in digital communications channels. They can, however, be negated by employing an equalizer in the receiver.
In many cases the amplitude level and the time delay of the reflections are time varying. These amplitude level and the time delay variations are associated typically with wireless transmission, where they may result from movement of objects which reflect the transmitted signals. In order to compensate for this type of noise, the receiver must track the channel variations and adapt the parameters of the equalizer accordingly.
A problem frequently encountered in full duplex digital data communication systems that employ a limited bandwidth channel is the presence of linear distortion introduced into the signal propagation path. The linear distortion commonly manifests itself as intersymbol interference (ISI) in the received symbol sequence. In order to reduce the effects of this distortion, it is common practice in the signal processing art to process the received signals by some form of linear and non-linear filter mechanism, such as a decision feedback equalizer (DFE).
The samples are then fed to a feed forward linear filter section. The feed forward linear filter comprises a delay line, i.e., zxe2x88x921, and each stage of which stores a respective symbol sample. The contents of the respective stages of the delay line are multiplied by respective weighting coefficients Wi and then summed in an adder, to yield a combined output. This output can be applied to a downstream decision feedback section, from which output data decisions are derived.
The output of the adder is adjusted by subtracting the output of the decision feedback section from the output of the adder. The effect of subtracting the output of the decision feedback section from the linear filtered section is to remove intersymbol interference due to previously detected symbols.
Note that data decision estimates are derived on a symbol by symbol basis by means of a symbol decision mechanism, such as a symbol slicer. The symbol slicer functions to slice the signal at equally spaced levels between reference levels for the received symbols. These output data decisions are then fed back to a linear delay line to remove intersymbol interference from future symbols. The contents of the respective zxe2x88x921 stages of the delay line are multiplied by respective weighting coefficients and then summed in an adder to produce a combined output to be subtracted from the output of feed forward section.
A residual error signal for adjusting the weighting coefficients of the linear section and the decision feedback section of the filter may be obtained by differentially combining data decision estimates at the output with the output of the summation block. In the ideal conventional DFE equalizer architecture the weighting coefficients Wi for the feed forward filter section are assumed to be one-sided. i.e., anticausal, and the last, or most delayed, tap Zxe2x88x921 of the delay line is typically the largest and is commonly referred to as the main tap, reference tap or the cursor tap. The current decision on the value of a received symbol is customarily considered to have its dominant energy contribution derived through this tap.
The weighting taps of the feedback section take on values equal to samples of the postcursor or xe2x80x98tailxe2x80x99 of the received symbol which follows as the symbol energy decays.
Since the classical DFE structure assumes that the number of taps or stages is infinite, practical realization requires truncating the lengths of the respective feed forward and feed back delay lines at some practical number of taps per filter. In order to prevent significant degradation of the signal, the number of taps selected for the feedback stage must be sufficient to span all significant samples of the signal at the point of ISI cancellation. The number of taps of the upstream stage is not as readily apparent.
Although this number is related to the precursors, it is not necessarily equal to the significant energy span of the precursors. One method to establish the length of the filter is to either compute the coefficients or simulate the filter with a large number of coefficients and determine how many are significant. This approach, however, is heavily channel dependent since, in practice, the signal processing circuit designer does not have the freedom to implement a xe2x80x98whitenedxe2x80x99 matched filter in the analog domain prior to sampling, which would be different for every line shape and noise spectrum. Ultimately, some prescribed fixed shaped is employed, or a simple anti aliasing filter may be used upstream of the sampling point.
In order to train the adaptive equalizer, data values or symbols corresponding to the transmitted data are used. Training is normally carried out using a predetermined training sequence. Alternatively, if the data decisions are sufficiently reliable prior to convergence, these data decisions can be used for training. When a training sequence is employed it is common practice to derive a rough approximation of the amount of delay and allow the taps to grow until the largest tap is identified. Then the amount of delay is adjusted so as to place the cursor tap at the desired location that is the last stage of the feed forward delay line.
Prior art solutions, such as that described above, are limited in their ability to track fast time variations in channels having reflections with large delays. To compensate for the linear distortion in such channels, an equalizer with a large number of parameters is needed, making fast tracking of channel variations difficult (and sometimes even not feasible). For example, a terrestrial digital television (DTV) signal may have reflections of up to 20 micro-seconds. If a linear equalizer or a decision feedback equalizer (DFE) is used to combat such reflections, then at least 200 taps will be required. These reflections may originate from a moving airplane, in which case they can vary significantly within a period of 10000 symbols, and thus be very difficult to track when using a prior art equalizer architecture.
This present invention is an adaptive equalizer structure and an equalization method that permits fast tracking of time varying reflections without sacrificing the stability of the equalizer. The ability to track fast variations in due in part to the sectioning of the equalizer into small filtering sections. The equalizer identifies sections of the equalizer that need to be adjusted rapidly due to channel variations and, consequently, the adaptation rate of the parameters of these sections is then increased.
Each equalizer section has an adaptive phase rotator (in QAM receivers) or an adaptive gain (in VSB or PAM receivers). One can show that the effect of reflection-delay variation when the signal is transmitted at a high RF frequency can essentially be compensated for by rapid adaptation of only the phase (or gain) parameter in the relevant sections, while all the other parameters of the equalizer do not need to be adjusted rapidly. This is also exploited by the equalizer to allow tracking fast channel variations using a relatively small number of parameters.
The adaptive equalizer is also used to construct a RF receiver. The equalizer is constructed from a plurality of feed forward equalizer (FFE) sections and a plurality of decision feedback equalizer (DFE) sections. A novel feature of the invention is that the equalizer is divided into a plurality of sections wherein separate taps and gain parameters are associated each section that can be set independently from the other equalizer sections. A controller monitors each equalizer section and in response thereto, adjusts the equalizer section parameters accordingly. This permits the equalizer to very quickly track reflections and other linear noise sources without sacrificing stability and jitter.
The outputs of the FFE and DFE sections are summed and input to an error calculator and a symbol slicer. The output of the symbol slicer is fed back into the first DFE section. The output of the error calculator is input to each FFE and DFE section and used in generating each respective output.
In operation, when reflections are detected, the parameters of only the section corresponding to the time span of the reflections are modified. The step size for the equalizer section corresponding to the reflection is made larger thus achieving faster tracking while the other equalizer sections remain unchanged or adapted at a slower rate.
An equalizer controller function determines the step size for each equalizer section. The controller assigns a large step size only to some of the equalizer sections while assigning the other sections a small step size. This provides a fast tracking rate for the sections associated with the reflection while maintaining stability and low noise thus optimizing the performance of the equalizer.
Each equalizer section is comprised of a plurality of delay line cells and a plurality of adaptive multipliers. Each delay line cell is associated with one adaptive multiplier. The outputs of the adaptive multipliers are summed and input to an adaptive multiplier that generates the data output of each equalizer section.
There is provided in accordance with the present invention an adaptive equalizer having an input and an output, the equalizer for equalizing a receiver signal input thereto comprising a plurality of feed forward equalizer (FFE) sections, at least one feed forward equalizer section having tap coefficients and step size parameters that are adjustable separately for each the feed forward equalizer section independent of the tap coefficients and step size parameters of other feed forward equalizer sections, a plurality of decision feedback equalizer (DFE) sections, at least one decision feedback equalizer section having tap coefficients and step size parameters that are adjustable separately for each the decision feedback equalizer section independent of the tap coefficients and step size parameters of other decision feedback equalizer sections, a summation unit operatively coupled to the outputs of the plurality of FFE sections and the plurality of DFE sections, the summation unit adapted to add the outputs so as to generate a soft decision output, a symbol slicer operatively coupled to the output of the summation unit and adapted to generate estimated symbols yielding a hard decision output, an error calculation unit operatively coupled to the output of the summation unit adapted to generate an estimated error signal and a controller operatively coupled to the plurality of FFE sections and the plurality of DFE sections, the controller adapted to generate the step size parameters for each FFE section and DFE section in response to sensing variations in the linear distortion of the receiver signal which can be compensated for by the FFE and the DFE sections.
The feed forward equalizer section comprises a linear adaptive digital filter operating on the input of the equalizer delayed in time and generating an output therefrom, the linear adaptive digital filter including a plurality of coefficients, an adaptive gain multiplier operating on the output of the linear adaptive digital filter, the adaptive gain multiplier including a gain coefficient, a first step size parameter for adapting the plurality of coefficients of the linear adaptive digital filter, a second step size parameter for adapting the gain coefficient of the adaptive gain multiplier and wherein the first step size parameter and the second step size parameter are set independently for each the FFE section.
The decision feedback equalizer section comprises a linear adaptive digital filter operating on the estimated symbols delayed in time and generating an output therefrom, the linear adaptive digital filter including a plurality of coefficients, an adaptive gain multiplier operating on the output of the linear adaptive digital filter, the adaptive gain multiplier including a gain coefficient, a first step size parameter for adapting the plurality of coefficients of the linear adaptive digital filter, a second step size parameter for adapting the gain coefficient of the adaptive gain multiplier and wherein the first step size parameter and the second step size parameter are set independently for each the DFE section.
The controller comprises processing means programmed to sense the magnitudes of gradient outputs generated by each the FFE section and each the DFE section and adjust the step size parameters within those FFE and DFE sections having high gradient magnitudes so as to allow fast adjustment of tap coefficients.
The controller comprises processing means programmed to monitors the tap coefficients associated with each the FFE section and each the DFE section, detect the time variation of the coefficients, normalize the time variation with the step size parameters for a section, adjust the step size parameters within those FFE and DFE sections having large time variations so as to allow fast adjustment of tap coefficients.
The controller comprises processing means programmed to set large values for the step size parameters in a FFE or DFE section that is expected to have large or fast time varying tap coefficients and set small values for the step size parameters in a FFE or DFE section that is expected to have small or slow time varying tap coefficients.
The tap coefficients of each FFE section are adapted in accordance with the cross correlation between the input to the FFE section and the estimated error signal. The gain coefficient of each FFE section is adapted in accordance with the cross correlation between the input to the adaptive gain multiplier and the estimated error signal. The tap coefficients of each DFE section are adapted in accordance with the cross correlation between the input to the DFE section and the estimated error signal. The gain coefficient of each DFE section is adapted in accordance with the cross correlation between the input to the adaptive gain multiplier and the estimated error signal. The tap coefficients in each FFE section are adapted such that they converge to a solution wherein the magnitudes of the tap coefficients are close to a predetermined value.
There is also provided in accordance with the present invention a communications receiver for receiving transmissions that are transmitted within a communications system comprising a sensor for receiving the transmitted signals, a front end unit adapted to receive the output of the sensor, the front end unit operative to downconvert, filter and amplify the transmitted signal so as to generate an baseband signal, an adaptive equalizer having an input and an output and adapted to receive the baseband signal, the adaptive equalizer comprising a plurality of feed forward equalizer (FFE) sections, at least one feed forward equalizer section having tap coefficients and step size parameters that are adjustable separately for each the feed forward equalizer section independent of the tap coefficients and step size parameters of other feed forward equalizer sections, a plurality of decision feedback equalizer (DFE) sections, at least one decision feedback equalizer section having tap coefficients and step size parameters that are adjustable separately for each the decision feedback equalizer section independent of the tap coefficients and step size parameters of other decision feedback equalizer sections, a summation unit operatively coupled to the outputs of the plurality of FFE sections and the plurality of DFE sections, the summation unit adapted to add the outputs so as to generate a soft decision output, a symbol slicer operatively coupled to the output of the summation unit and adapted to generate estimated symbols yielding a hard decision output, an error calculation unit operatively coupled to the output of the summation unit adapted to generate an estimated error signal, a controller operatively coupled to the plurality of FFE sections and the plurality of DFE sections, the controller adapted to generate the step size parameters for each FFE section and DFE section in response to sensing variations in the linear distortion of the receiver signal which can be compensated for by the FFE and the DFE sections, a demodulator operative coupled to the output of the adaptive equalizer and a detector operatively coupled to the output of the demodulator and adapted to generate binary output data.
There is further provided in accordance with the present invention an adaptive filter having an input and an output, the filter for filtering an input signal comprising a plurality of filter sections, at least one filter section having tap coefficients and step size parameters that are adjustable separately for each the filter section independent of the tap coefficients and step size parameters of other filter sections, a summation unit operatively coupled to the outputs of the plurality of filter sections, the summation unit adapted to add the outputs so as to generate a soft decision output, an error calculation unit operatively coupled to the output of the summation unit adapted to generate an estimated error signal and a controller operatively coupled to the plurality of filter sections, the controller adapted to generate the step size parameters for each filter section in response to sensing variations in the linear distortion of the input signal which can be compensated for by the filter sections.
The filter section comprises a linear adaptive digital filter operating on the input of the adaptive filter delayed in time and generating an output therefrom, the linear adaptive digital filter including a plurality of coefficients, an adaptive gain multiplier operating on the output of the linear adaptive digital filter, the adaptive gain multiplier including a gain coefficient, a first step size parameter for adapting the plurality of coefficients of the linear adaptive digital filter, a second step size parameter for adapting the gain coefficient of the adaptive gain multiplier and wherein the first step size parameter and the second step size parameter are set independently for each the filter section.
Still further, there is provided in accordance with the present invention an echo canceling device comprising an adaptive filter having an output, and an input adapted to receive an outgoing signal, the adaptive filter adapted to generate an estimated echo signal, the adaptive filter comprising a plurality of filter sections, at least one filter section having tap coefficients and step size parameters that are adjustable separately for each the filter section independent of the tap coefficients and step size parameters of other filter sections, a summation unit operatively coupled to the outputs of the plurality of filter sections, the summation unit adapted to add the outputs so as to generate a soft decision output, an error calculation unit operatively coupled to the output of the summation unit adapted to generate an estimated error signal, a controller operatively coupled to the plurality of filter sections, the controller adapted to generate the step size parameters for each filter section in response to sensing variations in the linear distortion of the input signal which can be compensated for by the filter sections and a summer adapted to sum together a first signal and a second signal so as to generate an echo free signal, the first signal comprising the sum of an echo generated by time varying reflections and an incoming signal source, the second signal comprising the output of the adaptive filter.